The utilization of single-mode fibers in high rate transmission systems has prompted the use of single-fiber optical attenuators therein, the attenuation function whereof is obtained by playing on the coupling parameters of a fiber-to-fiber junction, said parameters being the tilt angle of the two fiber ends and the distance between the two fiber ends or the two orthogonal components defining said distance.
Modifications made to these parameters either separately or, preferably, simultaneously, provide a variable attenuation according to a known law of variation.
The article entitled, "Variable attenuator for use in single-mode fiber transmission systems," published in the July 15, 1980 issue of Applied Optics (Vol. 19, No. 14, pages 2435 to 2438), deals with a variable attenuation rate obtained by combining these coupling parameters for a fiber-to-fiber junction. The basic idea of the attenuator described in this article consists in making the end of one of the fibers movable along a circular arc in an attenuation element defining the fiber-to-fiber junction of the attenuator. In the embodiment illustrated, the attenuation element comprises a support with a V-shaped groove on a concave surface, termed the "curved V-channel" for its circular arc shape, which receives the two ends of the two single-mode fibers defining the junction. On this support, the end of one of the fibers is fixed in the channel whereas the end of the other fiber is movable along the channel. The moving fiber is attached to the end of a pivotably mounted arm whose pivot is at the V-channel curvature center. The arm is driven by a precision micrometer head controlling the extent of its motion.
In this type of construction, both fibers are necessarily mounted in V-channels, for purposes of precision and of reproducibility of the attenuation values based on groups of values taken individually by the coupling parameters. The range of possible attenuation values as well as the accuracy and reproducibility of the attenuation values are thus limited by the very nature of this means of holding the fiber ends relative to one another and by the spurious reflections on the channel walls when the distance between the fiber ends is increased. Moreover, the precision and the reproducibility of the attenuation values, tied as they are to the precision channel machining, are also directly dependent upon the attenuator operating and handling conditions, which conditions may lead to damage of the fiber end moving in the channel and/or, in the event of shocks or vibrations being applied to or transmitted to the attenuator, to a fiber's partly or entirely leaving the V-groove, particularly the moving fiber, which is kept in the bottom of the groove by its springiness alone.
The present invention is directed to providing a variable optical attenuator of simple design which is easy to manufacture on an industrial scale and avoids the above-mentioned disadvantages.